Environmental concern has become one of the most important topics in the coatings industry in the last three decades. Researchers are continually attempting to develop “greener” coatings systems. Seed-oil based materials are an attractive choice, as they are biodegradable and readily available from renewable resources. Seed oils are triglycerides of fatty acids. Seed oils such as linseed oil, soybean oil, or tung oil can form a networked polymer film when exposed to air and are often used in the manufacture of coating binders.
Seed oils are classified into different categories (non-drying, semi-drying, and drying) based on the number of unsaturation sites located in their fatty acid side chains. The higher the number of unsaturation sites, the more readily a film is formed when exposed to the atmosphere. The process by which a seed oil based film is formed is commonly referred to as “auto-oxidative” curing. Oxidation of the drying oil begins when molecular oxygen attacks an active center on a fatty acid chain, followed by the hemolytic cleavage of the peroxide to produce free radicals. Hydrogen is abstracted from the methylene group between carbon-carbon double bonds followed by isomerization in a double bond position to form a conjugated structure. Termination results in carbon-carbon, ether, and peroxy linkages which create an interlocking network.
Alkyds, which are derived from plant and vegetable oils, provide an attractive alternative to more commonly used binders. Typical alkyd resins incorporate linseed oil, soybean oil, safflower oil, and other drying oils into their chemical makeup. Alkyd-based coatings have several advantages including high gloss, good color/gloss retention, good heat and solvent resistance, and an auto-oxidative crosslinking mechanism. However, they require the use of organic solvents, typically volatile organic solvents (VOCs), to reach the desired application viscosity, which presents a problem in light of environmental restrictions which have recently become more severe as government and industry attempt to respond to the need to be more environmentally friendly.
One possible solution to resolve this strict regulation problem is to develop new materials to be used as diluents. These materials function as an organic solvent in the coatings formulation, but are integrated into the film during the curing process. Tung oil has been used in efforts for new materials. Tung oil based material has been reported for use in radiocurable compositions (Poortere, D., Radiocurable Compositions, 1978, UCB). Additionally, a study employing cationic copolymerization of tung oil was also conducted (Li, F and R. C. Larock, Synthesis, Structure and Properties of New Tung Oil-Styrene-Divinylbenzene Copolymers Prepared by Thermal Polymerization. Biomacromolecules, 2003. 4(4): p. 1018-1025).
Tung oil is a popular choice as the foundation of new materials due to the difference in chemical makeup from most other materials in its class. Tung oil is derived from the nuts of Aleurites fordii. It is obtained from the kernels of the nuts and classified as a drying oil. Typical fatty acid compositions of tung oil are: 5% saturated acid, 8% oleic acid, 4% linoleic acid, 3% linolenic acid and 80% α-eleostearic acid. Due to its unique drying speed and excellent water resistance, tung oil is very valuable in modern manufacturing of varnish and related materials. Oxidation of conjugated tung oil catalyzed by metal driers has been investigated. It has been shown that besides radical recombination, crosslinking reactions also occur through direct addition of free radicals to the conjugated double bonds, frequently leading to higher molecular weight oligomers. Even in light of the foregoing, alkyd-based coating systems would benefit from enhanced properties, including reduced surface energy, increased thermal stability, low refractive index and friction coefficient, increased hydro- and lipophobicity, and others.